For 10 years they have not slept easy. The casual killers of the six cowed and beaten prisoners from Srebrenica were happy to play to the camera that day in July 1995, high on victory and heroes in the eyes of many fellow Serbs. But, as the years have worn on, that sheen has dimmed and the fear has grown. Did the tape still exist? Who had it? Where was it?

For the first time since the execution video was shown at the UN's war crimes tribunal for the former Yugoslavia in The Hague on Wednesday, The Observer can reveal the full story of the tape that has rocked not just Serbia and Bosnia, but the whole world.

It is the extraordinary story of how the tape was hidden for more than nine years but then, as its existence was revealed in a trial in Serbia, how a race began between the frightened killers and Serbia's leading human rights activist to find it - to destroy it or get it out to the world.

The tape is also the 'smoking gun', for it is the final, incontrovertible proof of Serbia's part in the Srebrenica massacres in which more than 7,500 Bosnian Muslim men and boys were murdered. Until last week Serbian officials, both from the wartime regime of Slobodan Milosevic and since his fall in 2000, have argued that Serbia was not involved with the massacres. Now, the tape proves that to have been a lie.

It will prove valuable ammunition, not just in trials like that of Milosevic but also in Bosnia's action at the UN's International Court of Justice in which it has charged Serbia with complicity in genocide. The gruesome tape shows the execution of six Bosnian Muslim prisoners, four of whom were under 18 and the other two under 30. The beaten prisoners, hands bound, are shown lying face down in a lorry. A guard kicks one in the head. They are ordered off the truck, told to lie down and, in a later clip, shot in the back while standing.

The first four to die are ordered to walk forward, one by one, and then shot. Then the hands of the last two are unbound and they are told to carry the bodies to another spot, where they are also shot.

The cameraman, known by his nickname, Bugar, is impatient. He wants his fellow Skorpions, a unit belonging to Serbia's Ministry of the Interior, to hurry because his camera battery is running low.

The murders took place close to the village of Trnovo, which lies 30 minutes' drive east of Sarajevo, which was then still a city besieged by Serbian forces. From the beginning of the war in 1992, Trnovo was in Serbian hands.

In the summer of 1995 General Ratko Mladic, the Bosnian Serb army commander, is determined to win the war. He needs to concentrate all his men around the east Bosnian Muslim enclave of Srebrenica, declared a UN safe haven. He plots a strategy. His own men will deal with the enclave, but men from Serbia will attack Sarajevo to create a diversion and to tie down Bosnian government troops. Enter the Skorpions. Since 1991 this unit has played a key role, especially in eastern Slavonia, that part of Serbian-held eastern Croatia abutting Serbia. It is headquartered in Sid, an unremarkable little town, 80km north-west of Belgrade, but which during the Croatian war became the jumping-off point for the Serbian attack on eastern Croatia.

Their job is special - 'black operations' - and they are also used by the Milosevic regime to make sure the local Serb authorities, especially in occupied eastern Slavonia, do as they are instructed. To keep them happy, Belgrade allows them a free hand in smuggling and looting.

It is now 26 June, 1995. The Skorpions are about to set off for Bosnia and, while dogs nose around, an Orthodox priest blesses the men who are wearing red berets and black jumpsuits. Next clip: they are in Pale, by the coaches which have brought them to the Bosnian Serb wartime capital. They pose for the camera in front of Pale's roadside name sign. Next clip: the executions.

What has been shown on Serbian and Bosnian TV is four minutes of a two-hour tape. As the Skorpions went into action this was, in effect, their Bosnian 95 tour video. Says Natasa Kandic, Serbia's leading rights campaigner, who gave the tape to Serbia's war crimes court and the one in The Hague, 'they filmed everything'.

Soon after arriving in Pale, the men were sent to Trnovo and from there they fanned out to launch their feint on Sarajevo. The plan worked, and on 11 July the defences of Srebrenica had collapsed. Now thousands of prisoners were falling into the hands of Mladic. Up on Mount Jahorina, overlooking Sarajevo, says Kandic, the Bosnian Serb leader, Radovan Karadzic, and his Vice-President, Nikola Koljevic, needed to work on distributing the prisoners. In Trnovo the Skorpions were ordered to send some of their vehicles to Srebrenica to collect their share of the Muslim captives. According to Kandic, who heads Belgrade's Humanitarian Law Centre and who has the story from the man who gave her the tape, a coach packed with more than 50 prisoners arrived in Trnovo.

At that point the only people in Trnovo were the Skorpion's commander, Slobodan Medic, then 28, and a few of his men. Medic, says Kandic, now got orders 'to take some people from the bus and kill them. He told his men to take six'. The six seem to have been taken by truck, up to a secluded spot close by called Godevinske Bare, where the killings took place. The coach with the other prisoners then continued to other locations and 'all the other prisoners were killed', says Kandic.

Bugar, the man filming, was a close friend of Medic. The day after the killings, which took place between 15 and 19 July, he gave the camera to someone else. When the unit came home to Sid that person made 20 copies, distributed among the then acclaimed Skorpion 'heroes'.

When he discovered this, Medic was furious. He knew the tape could be used as evidence if it fell into the wrong hands. He ordered everyone to return the tapes and 20 were destroyed. But one man, who had rowed with his colleagues and had left Trnovo the day before the executions, made one more copy for himself. Fearful and uncertain what to do, the man, whom Kandic cannot name but who will testify at The Hague, hid the tape outside Serbia. For years nothing happened.

That is, until 2003. As war broke out in Kosovo, the Skorpions were reactivated. In March 1999 they lived up to their reputation, killing 19 ethnic Albanian civilians in Podujevo. Two Skorpions were put on trial and in 2003, one of them, who agreed to testify against the others, mentioned that a tape of their Bosnia 95 tour existed. Kandic contacted the man, who said he did not have it but knew the man who did. She went to Sid and found him. He gave it to Kandic who agreed not to use it until he was out of the country.

In Sid, tension began to rise. Kandic had been spotted there with the man who had the tape and the other Skorpions guessed he had told her about the Srebrenica prisoners. A desperate hunt began. Kandic began to get reports that the Skorpions were attacking and harassing people in Sid as they searched for the cassette.

On 9 December, the Hague tribunal released on bail Frenki Simatovic, former head of the Red Berets, another Interior Ministry unit which had worked with the Skorpions, and Jovica Stanisic, former head of the Serbian secret police. The orders went out to find the tape.

In Serbia, with the tenth anniversary of the massacre coming up, Kandic, angry at claims in public by various personalities that Srebrenica had been 'liberated' and that there had been no genocide, said she had seen the tape.

On 23 May she gave it to Serbia's own war crimes prosecutor. He promised to investigate, but no arrests were made. She also gave it to The Hague's prosecution team, who showed it on Wednesday. Immediately afterwards she gave it to Serbian TV.

Within 24 hours four men had been arrested in Serbia, including Slobodan Medic. Six more were brought in for questioning but were later released. Three are on the run. Serbia's police dossier on the case contains 136 names.

For the Serbian authorities, a psychological barrier has been smashed. Pressure is now mounting on them to arrest at least Mladic, who is believed to be in Serbia. Svetozar Marovic, the President of Serbia and Montenegro, has said he will be arrested within a month. Boris Tadic, the President of Serbia, wants to attend the commemoration of the massacre at Srebrenica on 10 July.

Now, says Kandic, Serbia must arrest Mladic. 'After this,' she says, 'there is no choice. Serbs have been forced to see what happened and they have to stop denying that Serbia's forces were there.'